Understanding thread basics with Python

Recently, I needed to use the threads in order to increase the CPU efficiency in terms of idle time in Python. I came a cross great examples from agiliq's blog. I have reimplemented his code and add some minor comments.
You can refer the original post with more details.

Examining thread order:

'''
Created on Sep 29, 2013
@author: Bekoc::algorithms
'''

from threading import Thread
import time
import urllib2

class GetUrlThread(Thread):
    def __init__(self, url):
        self.url = url 
        super(GetUrlThread, self).__init__()

    def run(self):
        resp = urllib2.urlopen(self.url)
        print self.url, resp.getcode()

def get_responses():
    urls = ['http://www.google.com', 'http://www.amazon.com', 'http://www.ebay.com', 'http://www.alibaba.com', 'http://www.reddit.com']
    start = time.time()
    threads = []
    for url in urls:
        t = GetUrlThread(url)
        threads.append(t)
        print ('Thread %s is calling %s' %(t.getName(), url))
        t.start()
    for t in threads:
        t.join()
    print "Elapsed time: %s" % (time.time()-start)

get_responses()

Race Condition example without use of lock.acquire and lock.release:

'''
Created on Sep 30, 2013
@author: Bekoc::algorithms
'''
from threading import Thread
import time
#define a global variable
some_var = 0

class IncrementThreadRaceCondition(Thread):
    def run(self):
        #we want to read a global variable
        #and then increment it
        global some_var
        read_value = some_var
        time.sleep(.001)
        print "some_var in %s is %d" % (self.name, read_value)
        some_var = read_value + 1 
        print "some_var in %s after increment is %d" % (self.name, some_var)

def use_increment_thread():
    threads2 = []
    for i in range(50):
        t = IncrementThreadRaceCondition()
        threads2.append(t)
        t.start()
       
    for t in threads2:
        t.join()
    print "After 50 modifications, some_var should have become 50"
    print "After 50 modifications, some_var is %d" % (some_var,)

use_increment_thread()

in order to prevent the race condition change the run function  and import Lock:

from threading import Lock

    def run(self):
        #we want to read a global variable
        #and then increment it
        global some_var
        lock.acquire()
        read_value = some_var
        time.sleep(.001)
        print "some_var in %s is %d" % (self.name, read_value)
        some_var = read_value + 1 
        print "some_var in %s after increment is %d" % (self.name, some_var)
        lock.release()

Khan Academy offers Python Programming course

There are lots of Python online courses going on around, but take a look at khan academy playlist. It includes the basic programming concepts with Python.

1. Introduction to Programs Data Types and Variables
2. Binary Numbers
3. Python Lists
4. For Loops in Python
5. While Loops in Python
6. Fun with Strings
7. Writing a Simple Factorial Program. (Python 2)
8. Stepping Through the Factorial Program
9. Flowchart for the Factorial Program
10. Python 3 Not Backwards Compatible with Python 2
11. Defining a Factorial Function
12. Diagramming What Happens with a Function Call

1. Recursive Factorial Function
2. Comparing Iterative and Recursive Factorial Functions
3. Exercise - Write a Fibonacci Function
4. Iterative Fibonacci Function Example
5. Stepping Through Iterative Fibonacci Function
6. Recursive Fibonacci Example
7. Stepping Through Recursive Fibonacci Function
8. Exercise - Write a Sorting Function
9. Insertion Sort Algorithm
10. Insertion Sort in Python
11. Stepping Through Insertion Sort Function
12. Simpler Insertion Sort Function